Mexico, together with the peninsula of Lower California and a portion of the southern extremity of California, are arid, although in immediate proximity to the ocean. The leading characteristics of the climate of this, the arid portion of the tropical province, are its small rainfall, clear atmosphere, high mean annual temperature, moderate seasonal changes, and active evaporation.
The most typical portion of the arid region bordering the Pacific from California southward lies in northwestern Mexico, where the mean annual precipitation on the western slope of the central table-land is from 15 to 20 inches, but gradually diminishes as one descends to the lower lands to the westward to 10 inches, and even to 5 inches or less. A great portion of the lowlands is practically a desert, although, as is common on the desert-like tracts of this continent, it is sparingly covered with detached clumps of cacti, yuccas, and other similar plants which live with the minimum of water.
The hot, arid lands of the southwest just referred to are similar in position, in reference to the relation of land areas to the trade-wind belts, as other trade-wind deserts, as they are termed, such as the Sahara, which occur on the leeward side of continents where they are crossed by these drying winds.
The Lower Austral Province (Plate III).—This province embraces the Atlantic and Gulf coasts of the United States from Connecticut southward, with the exception of the tropical portion of southern Florida; the Mississippi basin south of the mouth of the Ohio; and includes also many of the valleys amid the Pacific mountains from the northern boundary of California to central Mexico. The larger geographical conditions on which the boundaries of the province depend are the warm currents in the Atlantic, which ameliorate the temperature of the adjacent land as well as supply it with abundant moisture; the low elevation of the central continental basin, which permits of great extension northward of the climatic conditions originating about the Gulf of Mexico; and the mountainous character of the western portion of the province, where the valleys
have a markedly different temperature and rainfall from the highlands.
The most southern portions of the lower austral province within the border of the United States has a semitropical climate and is characterized by its high mean annual temperature, abundant rainfall, and uniformity of weather conditions throughout the year. The summers are hot, but tempered by winds from the Gulf of Mexico and the Atlantic, and the winters, although normally mild and without snow, are varied at intervals by periods of cold which bring occasional frosts. This semitropical division of the lower austral province embraces a narrow strip of coast-land extending from South Carolina to southeastern Texas, with the exception of the extreme southern end of Florida, and forms a transition between the hotter and more humid tropical province to the southward and the cooler and less humid region to the northward.
As the saying is, a tree is judged by its fruit; in a similar way we may judge of the climate of a region by its products. The Gulf strip of the lower austral province is the home of a number of plants and animals not found farther north, as, for example, the cabbage palmetto and Cuban pine, and several species of birds and small mammals. Among the agricultural products of this narrow coastal belt which are suggestive of its climate are rice, sugar-cane, and sea-island cotton; of fruits it produces, especially in Florida, oranges, mandarins, lemons, limes, shaddocks, and pineapples.
The humid portion of the lower austral province extends northward from the Gulf strip, and presents a transition in climatic conditions between the semitropical Gulf coast and the more markedly temperate climate of the upper austral. This eastern division of the lower austral, characterized by its mild winters, general absence of snow, long hot summers with abundant rain, extends from eastern Virginia southward about the southern end of the Appalachian Mountains, and in the Mississippi Valley broadens so as to reach the mouth of the Ohio River. Its western limit is in central Texas, where the mean annual rainfall decreases
to less than 20 inches, and forests give place to treeless, grass-covered plains and plateaus.
Through this eastern portion of the lower austral the mean annual precipitation is in general between 50 and 60 inches, and is so distributed throughout the year that each month receives approximately an equal share. In general, however, the rainfall is greater in winter than in summer. The mean annual temperature of the humid portion of the lower austral is from 60° to 68° F., the mean winter temperature 40° to 52°, and the mean summer temperature from 75° to 80° F. From these records it may be inferred that the conditions are favourable for the growth of trees. In its natural condition this entire region was clothed with a varied and beautiful forest, consisting largely of broad-leaved trees, but is also the home of the southern pine and the cypress. The characteristic crops are cotton and corn (maize).